Underwater navigation has typically been accomplished using inertial navigation or acoustic beacons. Acoustic systems are degraded by noise and interference from a number of sources. They are also subject to multi-path effects and in some environments are virtually unusable. Inertial navigation systems are complex, bulky, high cost, accumulate inaccuracy over time and require knowledge of an initial reference point.
U.S. Pat. No. 6,865,139 describes a sub-sea navigation system that uses electromagnetic transmission. This has a plurality of antennas located at known positions on a sub-sea structure. Each antenna is electrically coupled and includes a cathodic protection anode. Signals emitted by the antennas are used by a detection means on a sub-sea vehicle to allow the vehicle to navigate relative to the sub-sea structure. The detection means uses a measure of the electric field of the emitted signals in order to determine the position of the vehicle. A problem with the system of U.S. Pat. No. 6,865,139 is that the signals emitted by the electrically coupled antennas are subject to high near field attenuation and the receive antennas have low efficiency. This reduces the range over which position can be determined and limits the applicability of the system. Also, there is little information provided on how exactly the position is determined using the measure of electric field.